ærial, as being the distinguishing parts of the air, taken in the stricter sense of the term. These particles have an elasticity resembling the spring of a watch. Elasticity is an essential property of the air, and it is thought no other fluid has any thing of it, only as it participates of air, or has air contained in the pores of it. Our air abounds with particles which being pressed by the weight of the incumbent atmosphere, or any other body, endeavour to free themselves from that pressure, by bearing against the bodies that keep them under it; and as soon as the removal of these bodies gives them way, they expand the whole parcel of air which they composed. Mr. Boyle found, that one and the same portion of air may take up 52,000 times the space it doth at another time. He found, that the same quantity of air, by only having the pressure of the atmosphere taken off in the pneumatic engine, and without increasing the spring with any adventitious heat, would possess above 13,000 times its natural dimensions. Dr. Gregory proceeds, That accordingly a globe of air, of one inch diameter, would at the distance of the semidameter of the earth from the earth, fill all the planetary regions much beyond the sphere of Saturn. Admirable rarefaction! The weight of air was discovered first by Galilæus, who found that water could not by pumping be raised higher that 34 or 35 feet. Torricellius afterwards pursued and improved the thought, and as a further proof of the weight of the air, invented that which we call the Torricellian experiment. Mr. Boyle found by repeated experiments, that the weight of air to water is as 1 to 1000. Dr. Halley rather determines the specific gravity of air to water, to be about 1 to 800. Mercury is to air as 10,800 to 1. And so, a cylinder of air, of 900 feet, is equal to an inch of mercury. We will, with Dr. Wainwright suppose a cubical foot of water to weigh 76 pounds troy weight. The compass of a foot square upon the superficies of our bodies, must sustain a quantity of air, equal to 2610 pounds weight. If the superfices of a man's body contains fifteen square feet, which is pretty near the truth, he will sustain a weight equal to 39,900 pounds troy. The difference between the greatest and the least pressure of the air upon our bodies, is equal to 3982 pounds troy. On which the doctor says, "No wonder then we suffer in our health by change of weather; it is surprising that every such change does not entirely break the frame of our bodies to pieces, and be the constant harbinger of sudden death." My God, it is because I have obtained help from thee, that I continue to this day! Sir Isaac Newton thinks true and permanent air to be made by fermentation and rarefaction of bodies that are of a very fixed nature. And it is plain, those particles fly and avoid one another with the greatest force at a distance, which when they are very near, do attract and adhere with the greatest violence. The particles of true and permanent air, being extracted from the densest and most fixed bodies, will be more dense and crass than those of vapour, and the parts of a humid atmosphere may be lighter than those of a dry one, as in fact they appear to be. He thinks therefore, that the rarefaction and condensation of the air cannot be ac counted for from the spring, or elastic forms of the particles, without a supposition that they are endued with some centrifugal force or power, by which they fly and avoid one another, and the dense bodies from which they are extracted. This may be the cause for ascent of water in small capillary tubes, to a much greater height than the surface of the water in the open vessel in, which they are placed. The air within the tubes is much rarer than in more open spaces, and presses less on the surface of the water within the tubes than without. It is admirable to consider the necessity of air to the whole animal world; how soon the vital flame languishes and expires if air be withheld from it! even the inhabitants of the water cannot live without it. It is evident the air easily penetrates water exposed to it, and diffuses itself through every part of it. Put fishes into a vessel of a narrow mouth, full of water, they will continue to live and swim there whole months and years. But stop the vessel, so as to exclude the air, they will suddenly be suffocated; an experiment often made by Rondeletius. Insects need more air than other creatures, having more air vessels for their bulk, and many orifices on each side of their bodies for its admission, which if you stop with oil or honey, they presently die. Malpighius has discovered and demonstrated, that the plants themselves have a kind of respiration, being furnished with vessels for the derivation of air to all their parts. Dr. Hulse, and Mr. Ray, and others, have now also rendered it very evident, that the fætus in the womb receive a measure of air from the maternal blood, by the placenta uterina, or the cotyledons. When this communication is broken off, what is it that now, to preserve the life of the animal, speedily raises the lungs, and fetches into them an abundance of air, which causes a sudden and mighty ascension in the blood, for the maintenance whereof a far greater quantity of air is requisite ? Certainly some intelligent being must now interpose, to put the diaphragm, and all the muscles that serve to respiration, into their motion! My God I know thee! And now, as our ingenious Waller sings; "Thus wing'd with praise, we penetrate sky, May sing together though we dwell apart." "The Syrians worshipped the air as a god. I will worship him that created it. I will give thanks to the glorious God, for the benefits with which the air is replenished by his bounty. It was long since called the paranymph, by which the espousal and communion between heaven and earth is carried on. I breathe in the favours of God continually. An ungrateful wretch, if I do not breathe, out his praises! How justly might the great God fill the air with such pestilential poisons, as might suffer the unholy and unthankful to breathe no longer in it !" ESSAY XVIII. Of WIND. WHAT better definition of wind, than a stream of the air? Plato long since defined it, the motion of the air about the earth. Other hypotheses for this current of the air not well answering all phenomena, the learned Mr. Halley recommends this to consideration, as the cause of it; the action of the sunbeams on the air and water, as the sun passes every day over the oceans, considered with the nature of the soil, and the situation of the continents adjoining. According to the laws of statics, the air, which is less rarefied and expanded by heat, and consequently more ponderous, must have a motion round those parts thereof, which are more rarefied and less ponderous, to bring it to an equilibrium. The presence of the sun also continually shifting to the westward, that part to which the air tends, by reason of the rarefaction made by his greatest meridian heat, is with him carried westward, and consequently the tendency of the whole body of the lower air is that way. Thus a general easterly wind is formed. From this principle, the easterly wind on the north side of the equator, should be to the northward of the east; and in south latitudes, it should be to the southward thereof: inasmuch as near the line, the air is more rarefied than at a greater distance from it. Here all the phenomena of the general trade winds are answered for; which if the whole surface of the globe were sea, would undoubtedly blow all round the world, as they are found to do in the Atlantic and Ethiopic oceans. But since great continents interpose, and break the continuity of the oceans, regard must be had to the nature of the soil, and the position of the high mountains, which cause the variation of the winds, from the general rule that has been proposed. If a country, which lies near the sun, prove to be low, flat and sandy, the heat occasioned by the reflection and retention of the sunbeams there, will so rarefy the air, that 9 |